The use of negative feedback in an electronic circuit generally produces changes in the characteristics of the circuit that improve the performance of the circuit. Negative feedback may be employed within an amplifier circuit to produce more uniform amplification, to stabilize the gain of the circuit against changes in temperature or component replacement, to control input and output impedances or to reduce noise or interference in the amplifier.
Feedback can be introduced into an amplifier by providing to the input of the amplifier a fraction of the amplifier output. A block diagram of a classical amplifier circuit including negative feedback is illustrated in FIG. 1. The amplifier circuit includes an amplifier 12, a feedback circuit 14, and a summing junction 10. The input signal, identified as X.sub.IN, is received by summing junction 10, combined with the output of feedback circuit 14 and provided to amplifier 12. The output of the amplifier circuit, identified as X.sub.OUT, is: EQU X.sub.OUT =A(X.sub.IN -.beta.X.sub.OUT); EQN 1
where:
A = the gain of amplifier 12; and
.beta. = the gain of feedback circuit 14.
The transfer characteristic, often referred to as the feedback gain A.sub.f, of the amplifier circuit is: EQU A.sub.f =X.sub.OUT /X.sub.IN =A/(1+A.beta.). EQN 2
In the limiting case, as A becomes very large, the transfer characteristic can be approximated by the following equation: EQU A.sub.f =1/.beta.. EQN 3
In the above equations the input and output signals, X.sub.IN and X.sub.OUT, respectively, can be either voltage or current signals. An amplifier which converts an input voltage signal into an output current signal is known as a voltage-to-current converter. Voltage-to-current converters may be utilized within drives for DC brushless motors or voice coil type motors, such as are employed in computer disk drives.
A typical voltage-to-current converter, also known as a voltage-controlled current source, built with standard analog components is shown in FIG. 2. The converter includes an operational amplifier OA the output of which is connected to the gate terminal of an N-channel MOSFET transistor M. A voltage input signal V.sub.IN is provided to the non-inverting input (+) of operational amplifier OA and a feedback voltage signal V.sub.F is provided to the inverting (-) input of operational amplifier OA. The drain terminal of transistor M is connected through a load (not shown) to a first reference voltage source V.sub.DD and the source terminal of transistor M is connected through a current sensing resistor having a resistance of R to a second reference voltage source V.sub.SS. The output current generated by the converter is identified as I.sub.OUT. The voltage developed across the current sensing resistor is provided to the negative input of operational amplifier OA as the feedback voltage signal V.sub.F. The feedback factor or gain, .beta., for the feedback function for the circuit shown in FIG. 2 is R (V.sub.F =I.sub.OUT *R).
The transfer function for the voltage-to-current converter, developed from EQN 2 by replacing .beta. with R, X.sub.OUT with I.sub.OUT and X.sub.IN with V.sub.IN, is therefore: EQU I.sub.OUT =V.sub.IN /R EQN 4
As stated above, feedback is provided to the operational amplifier by incorporating a current sensing resistor in series with the load and sensing the voltage developed across the sense resistor. Unfortunately, placing a sensing resistor in series with the output limits the compliance voltage of the converter, i.e. the voltage drop required to be developed across the current source in order to provide a current at the output of the current source. The sense resistor is also a source of power dissipation which is not attributable to the load.